 West Nile virus infection is a mosquito-borne zoonotic disease that is now endemic in the United States. During the next 50 minutes we're going to be talking about the history of the disease in the United States and around the world, some characteristics of the virus, and some discussion concerning how the virus may be transmitted from place to place. We'll be discussing human infections, avian infections, equine infection by West Nile virus, and some prevention and control measures that can be done. West Nile virus is endemic in Africa, Asia, Europe, and the Middle East. In the 1950s it was determined that up to 40% of Egyptians living in the Nile Delta had antibodies against West Nile virus. But in recent years there have been a number of new outbreaks. In 1996 there was an outbreak in Romania. In the following year both Israel and the Czech Republic had outbreaks of West Nile virus in the human population. By 1999 the virus had found its way to the United States and southern Russia. Prior to 1996 human disease was considered to be a minor aspect of West Nile virus infection. Very few people got sick with this disease, and very few avian mortalities occurred as a result of West Nile virus infection. But an outbreak in Bucharest, Romania, signaled a change in the West Nile virus genome that resulted in increased pathogenicity for both humans and for birds. In 1996 in Bucharest 400 people became sick with this virus and 40 people died. In the following year in Israel avian mortality was observed as a result of West Nile virus infection. And in 1999 in southern Russia 1,000 people became sick and 40 people died. As of now West Nile virus has moved essentially across all of the North American continent. The virus is present in southern Canada, across the United States, and already in early 2003 the virus has been reported in birds in Mexico. This slide shows the position of West Nile virus in southern Canada, shown in yellow, and throughout the United States. The virus has essentially moved all across the North American continent. The initial outbreak of West Nile virus in the United States occurred during the summer of 1999 in the New York City area. 7 people died in 1999 as a result of that initial outbreak. By 2001 27 states and the District of Columbia reported outbreaks of West Nile virus. Between 1999 and 2001 149 human illnesses were recorded. In the year 2002 a drastic change occurred. The virus was reported in 44 states plus the District of Columbia. Last year in 2002 3,389 people were reported to be sick and 241 human deaths resulted from the infection. Surprisingly enough it has been determined that hot dry summers actually promote the outbreak of West Nile virus infections in the United States. This slide shows the West Nile virus activity in this country between 1999 and 2002. As you can see by the cross-hatched area in 1999 the virus was present in the New York City area and in states immediately above and below New York State. By 2000 the virus had moved into Pennsylvania, New Hampshire, Virginia and North Carolina. By 2001 the virus spread into the Midwest including Iowa, Wisconsin, Illinois, Indiana and Michigan. By last year West Nile virus was reported in almost all states in the country including California on the West Coast. There has been much speculation as to how this virus entered the United States. One possibility is that migratory birds brought West Nile virus to this country. Some birds breed in Europe and winter along the eastern seaboard of the United States and move across the Atlantic Ocean in the fall to reach their wintering grounds. These birds include Wigeons, Greenwing Teal, Tufted Ducks and Gulls. Another possibility is that West African seaboard birds were blown by storms across the Atlantic Ocean to our shores. This occurrence has been documented with gray herons and egrets. And thirdly infectious mosquitoes either in the adult form or as eggs or larvae could possibly have been in an airplane and came to this country by transatlantic airplane flights. It has been speculated that West Nile virus first came to Israel by migratory birds. In Europe white storks normally hatch on the European continent in the spring and spend the winter time in Africa. And in the fall in the movement from Europe to Africa they migrate over the Middle East. In the fall of 1998 some of the white storks were forced down to the ground in Israel because of poor weather conditions. And it was determined that these white storks were carrying West Nile virus. Another bit of evidence that supports the movement of the virus with migratory birds is that following the initial outbreak in the New York City area in 1999 the virus was observed to move northward from New York City in the spring as the birds migrated northward. And in the fall as birds started migrating south the virus moved from New York City south. The virus itself is classified as a flavor virus. It is spread by arthropods primarily mosquitoes. The virus was initially isolated from an adult woman in the West Nile district of Uganda in 1937. Interestingly the Romanian virus first identified in 1996 has been found to be genetically very similar to the West Nile virus isolated in Israel in 1998. And the Israel virus is virtually identical to the virus that came to this country in 1999. The flavor virus that causes West Nile infection has a membrane envelope around it in a core structure 30 to 35 nanometers in diameter. It's an RNA virus with two different membrane proteins that are important to the replication of the virus. Two different lineages have been identified for West Nile virus. Lineage one causes most of the human encephalitis and is the form we have in the United States, Asia and Europe. Lineage two is found in Africa. A number of animals are susceptible host to West Nile virus including birds, mammals, reptiles and amphibians. Birds are considered to be reservoir hosts capable of amplifying the virus. Birds develop high levels of virus in the bloodstream and a minimum of 10 to the 5th virus particles per milliliter is considered necessary for viruses to be picked up by a mosquito and transferred to another host. And several species of birds as we'll see a little later have greatly exceeded the 10 to the 5th minimum threshold for serving as an amplifying host. Mammals usually develop a low viremia and are therefore called dead-in hosts because mosquitoes cannot pick up enough virus from a mammal to transfer the infection to another host. Reptiles as we have indicated are susceptible and a little later we'll talk about an outbreak that occurred in an alligator farm in Florida. Transmission is by arthropod vectors and in particular bird-loving mosquitoes or ornithophilic mosquitoes. Over 40 different species of mosquitoes have been found to carry West Nile virus and most of these belong to the genus Culex or Aedes. Ticks also have been known to carry the virus but ticks are considered to be a relatively minor transmission vector. Birds serve as a reservoir because of the high viremia that develops in birds and mammals are dead-in hosts because of the relatively low levels of virus that develop in the blood. In Iowa we have several other arthropod-born diseases in addition to West Nile virus. Another flavor virus, St. Louis encephalitis virus is found in Iowa and two toga viruses. Eastern equine encephalomyelitis virus and western equine encephalomyelitis virus occur in the in the state of Iowa. One other arthropod-born virus in Iowa is the lacrosse encephalomyelitis virus and this is classified as a bunya virus. All of these viruses are transmitted by mosquitoes. This slide shows that all of the viruses are present in mosquito. The principal arthropod vector is a mosquito and for all of these viruses certainly the first four wild birds play a role as amplifying hosts. For the lacrosse virus it's believed that small mammals such as squirrels are the primary amplifying hosts. For West Nile virus crows and blue jays are well-known indicator species because of high mortality in those species and also horses and we've had numerous episodes of West Nile virus infection in horses in recent years. West Nile virus has caused human infections in Iowa. With West Nile virus for people who are infected the case fatality rate varies between 3 and 15%. It has been estimated that between 140 and 300 people are infected with West Nile virus and show absolutely no symptoms of disease for every one person that is infected and becomes ill. The transmission cycle of West Nile virus begins with mosquitoes. Mosquitoes represent the arthropod vector most likely to transmit the virus. The virus can go from an infected mosquito to a wild bird and from a wild bird that develops high levels of virus in the blood it can go back to the mosquito again when the mosquito takes a blood meal. The mosquito may then bite another bird or a mammal such as a horse or a human. Mammals are dead-end hosts because they have a relatively low number of virus particles in their blood and the virus is not picked up by mosquitoes from mammals because of this low level of virus. In areas where West Nile virus exists less than 1% of people get infected. Of those that are infected less than 1% of the people get sick. West Nile virus causes two clinical forms of disease in humans. The first is called West Nile fever and about one-fifth of people who get sick have West Nile fever. This clinical form is manifested by fever, headache, muscle soreness, sore throat and a rash. 70% of people infected with West Nile virus that develop illness develop a meningoencephalitis. These people have serious neurological illness and the fatality rate is between 5 and 14%. A few people will develop an acute flaccid paralysis. This syndrome is relatively rare and is similar to what develops in some people who have been infected by poliomyelitis virus. This slide shows the areas in the United States where people were infected during the year 2002. For each state the number of infected people is shown on the map. Last year in Iowa we had 48 people sick with West Nile virus. Minnesota had 46. If we go a little bit further east we note much higher rates of infection. Illinois had 776 illnesses due to West Nile virus in 2002. 523 illnesses were reported in Michigan and 284 in Indiana. This slide shows the time of the year when most of the West Nile virus infections occurred in humans. As you can see the vast majority of infections occurred during August and September. A few still happened during the month of October but as the weather got colder and we developed frost West Nile virus infection of people essentially disappeared for the year. This slide again shows the two clinical forms of West Nile virus in people. West Nile meningoencephalitis that causes neurological problems and West Nile fever. Of people developing meningoencephalitis 59 was the average or the median age. For people developing West Nile fever 48 was the median age. So the central nervous system form of the disease tends to occur in older individuals. There are other ways that people can become infected with West Nile virus. It has now been documented that people have been infected through blood transfusions and organ transplantations. There is one documented case in New York State of an intrauterine infection to an unborn child. Laboratory workers are at risk. In one situation a laboratory worker was removing the brain from an infected bird and cut his skin with a scalpel blade and became infected. In two other episodes laboratory workers punctured their skin with contaminated needles. The susceptibility of birds to West Nile virus varies markedly with the age of the bird. In general juvenile or young birds are much more susceptible and experience high death rates and high levels of virus in the blood. Adult birds tend to be infected, develop antibodies, and have very low levels of death rate. Adult birds also tend to have much lower levels of West Nile virus circulating in the bloodstream. This slide shows that 10 to the 5th virus particles per milliliter of blood or 100,000 virus particles is the level required for an infectious viremia. An infectious viremia has sufficient virus particles in the bloodstream so that a mosquito may take a blood meal and pick up virus particles and transmit the virus to another host. In 2002 over 14,000 birds were found to be dead and contain West Nile virus in the carcasses. 7,700 of these birds were crows. About 5,000 blue jays were found dead last year. Individual mortalities are observed and the accumulated mortalities result in high levels of mortality in an area. In addition to crows and blue jays, 92 other species of birds were infected in 2002. Dr. Nicholas Comar at the Centers for Disease Control Laboratory in Fort Collins, Colorado has developed an index of reservoir competence for wild birds to measure the ability of these birds to serve as reservoir hosts. This formula involves the susceptibility of the bird, multiplied by the proportion of the population infected, multiplied by the duration of the infection. And the result is a numerical number reflecting the ability of that species to serve as a reservoir for a West Nile virus. 5 passerine birds were found to have the highest reservoir of competence index and are most able to transmit the virus to other mosquitoes. Blue jays had a reservoir competent index of 2.55 and had up to 10 to the 12th virus particles per milliliter of blood. 10 to the 12th represents an enormous number of virions in the blood. Grackles had a reservoir competence index of 2.84 and had 10 to the 11.8 virus particles per milliliter of blood. Housefinches, which we commonly see all across the United States, had a high reservoir competence index and had 10 to the 8.8 virus particles per milliliter of blood. Crows, which have experienced high mortality, also had a high reservoir competence index and high levels of virus particles in the bloodstream. Most sparrows found in most people's backyards and all across the United States also were found to be excellent reservoirs for West Nile virus and had high levels of virus in their bloodstream. In experimental infections, 3 out of 4 blue jays died due to West Nile virus or a mortality rate of 75%. 2 out of 6 experimental grackles were found to have a fatal infection with a mortality rate of 33%. Dr. Comar and his group in Fort Collins found that 2 out of 2 housefinches died, 8 out of 8 crows and 3 out of 6 house sparrows. Each of these 5 species of passerine or perching birds are considered to be excellent reservoirs for the West Nile virus. This slide shows the American crow, which has been experiencing high mortality from West Nile virus. And also on this slide we see a blue jay, another indicator species that when mortality occurs within blue jays, West Nile virus should be considered to be in the area. At the other extreme for reservoirs for West Nile virus is the Canadian goose, the pigeon, Bob White quail, ring net pheasants and the American coot. All 5 of these birds have a reservoir competence index near zero and the levels of virus in their bloodstream is much lower than the passerine birds that we talked about previously. Overall, in the United States it has been determined on the basis of annual breeding bird surveys and Christmas bird counts that there has been a significant decline in songbirds in our country. Last year in 2002 there was a dramatic increase in raptor mortality in the United States, in particular red-tailed hawks and great horned owls experienced high levels of mortality. This shows a red-tailed hawk and many of the raptor rehabilitators have reported high mortalities in the birds that they have cared for. West Nile virus also has the potential for infecting domestic poultry. Turkeys are of concern to us. Iowa is a state that raises about 8 million turkeys a year. In Israel in 1997 and 1998 it was observed that natural infections of turkey flocks occurred. These turkeys showed no disease and no death loss but they did develop antibodies indicating that the flock had been infected. Dr. David Swain at the Federal Laboratory in Athens, Georgia experimentally inoculated three-week-old turkey poles. These birds again developed no disease and they did develop antibodies indicating that infection had occurred. They had low levels of virus in the blood and they were found to shed the virus in their feces between four and seven days after the initial infection. No other turkeys in direct contact with the birds that had been inoculated developed West Nile virus infection. As a result of this research it is believed that turkeys are an unlikely amplifying host for West Nile virus. Chickens are also potential sources of host for West Nile virus. It has been determined experimentally in young chickens, chickens between one and eleven days of age, that they are readily susceptible to infection and they do develop high levels of virus in the blood up to 10 to the 6.3 virus particles per milliliter of blood. These young chicks may be amplifying host and they have the ability then to efficiently infect mosquitoes. In another experiment done by Dennis Sene and coworkers at the National Veterinary Services Laboratories in Ames, Iowa, seven-week-old birds were infected. These birds had no disease, relatively low virus levels, and did develop antibodies indicating recovery from infection. However, one chicken, four days after it was initially infected, developed virus levels of 10 to the 5 virus particles per milliliter, indicating that this chicken potentially could be a source of infection for mosquitoes and possibly be an amplifying host. So there may be a small window of opportunity for chickens to serve as amplifying host. Older chickens in Israel also were naturally infected in 1997 and 1998. Like the turkey flocks, these birds did not get sick, they did not die, but they did develop antibody titers. In another study with chickens done by Langevin and the coworkers at the CDC in Fort Collins, Colorado, chickens between 70 and 60 weeks of age were infected. These birds again had no disease but did develop antibodies. The virus levels on the average were less than 10 to the 4th or lower than the levels required to infect mosquitoes. However, one bird two days after it was infected did have a viremia of 10 to the 5th virus particles per milliliter, again in fact indicating that chickens during a narrow window of time might be a potential amplifying host. These workers also observed that within 24 hours after feces were deposited, 99% of the virus particles in the feces were dead. Ingestion of feces by other birds, sharing pins with the infected birds, did not result in transmission. So once again, research would suggest that chickens, especially older chickens, are unlikely to be a source of infection that mosquitoes could bite and transmit the virus to other hosts. In Israel it was noted that a flock of geese, in fact several flocks of geese between 3 and 12 weeks of age, developed high mortality and a number of these birds became paralyzed. It was determined that these observations in Israel warranted further research. David Swain in the Federal Laboratory in Athens, Georgia experimentally inoculated two-week-old goslings. These goslings did develop neurological disease and myocarditis. They did develop high levels of virus in the blood so that they could efficiently infect mosquitoes and geese are considered to be an amplifying host based upon the observations in Israel and the experimental observations in the United States. Equine infection was very much a problem throughout the United States in 2002. Last year over 14,500 horses were reported sick in 40 different states. These horses showed convulsions, paralysis, altered behavior and other indications of central nervous system disease. Most horses became sick between mid-August and the end of October. There is a vaccine available for horses to vaccinate against West Nile virus and this vaccine is produced by Fort Dodge Laboratories in Fort Dodge, Iowa. This slide shows the equine outbreaks that were reported in Iowa in 2002. As you can see, every county in Iowa had at least one horse with West Nile virus infection. Story County in Central Iowa where Iowa State University is located had 36 positive horses. And the indication is that based upon the illness in horses, West Nile virus has now spread to every county in the state of Iowa. Other animals also are susceptible to West Nile virus infection. West Nile virus has been found in bats, chipmunks, raccoons, skunks, rabbits and squirrels. Dogs and cats have been infected. In 2002 a mountain goat in Wyoming was found to be positive for West Nile virus and in Nebraska sheep and goats were found to be infected with this virus. In Minnesota an alpaca was positive for West Nile virus. As mentioned previously reptiles are also susceptible. In 2002 in Florida a 9,000 alligator farm had an outbreak of West Nile virus. The alligators showed evidence of neurological disease and very high levels of virus were found in their bloodstream up to 10 to the 6.5 virus particles per milliliter. Based on the high viremia it is reasonable to assume that alligators could efficiently infect mosquitoes and serve as an amplification host. It is important that people and animals be protected against West Nile virus infection. Reducing mosquito exposure is the first step in protecting against this disease. Insect repellents containing DEET should be used when persons and animals are likely to be exposed to mosquito bites. Many communities have initiated mosquito spraying programs. As of yet there is no human vaccine available for the human population. It is important to note that insecticides containing DEET should not be sprayed directly on dogs and cats. The higher the concentration of DEET in an insecticide the longer the protection lasts. It does not mean the protection is better but it will last longer. A formula containing 6.65% DEET will last for about 2 hours in terms of giving protection from mosquito bites. 20% DEET formulations provide about 4 hours of protection and 23.8% DEET insecticides will protect humans and mammals for about 5 hours. In conclusion it should be remembered that West Nile virus is an important disease for both people and for animals.